The present invention relates to cyanide-free copper plating baths. More particularly, the present invention relates to purification of cyanide-free copper plating baths whereby useful constituents of the copper bath may be precipitated and removed for reuse thereby leaving the contaminants for disposal.
The use of cyanide salts in copper plating electrolytes has become environmentally disfavored because of ecological considerations. Accordingly, a variety of non-cyanide electrolytes for various metals have heretofore been proposed for use as replacements for the well-known and conventional commercially employed cyanide counterparts. For example, U.S. Pat. No. 3,475,293 discloses the use of certain diphosphonate complexing agents for electroplating divalent metal ions; U.S. Pat. Nos. 3,706,634 and 3,706,635 disclose the use of combinations of ethylene diamine tetra (methylene phosphonic acid), 1-hydroxy, ethylidene-1,1-diphosphonic acid (HEDP), and amino tri(methylene phosphonic acid) as suitable ligands for the metal ions in which the bath further contains at least one strong oxidizing agent; while U.S. Pat. No. 3,928,147 discloses the use of an organophosphorus chelating agent for pretreatment of zinc die castings prior to electroplating with electrolytes of the types disclosed in U.S. Pat. Nos. 3,475,634 and 3,706,635.
While the electrolytes and processes disclosed in the aforementioned U.S. patents have provided satisfactory electrodeposits under carefully controlled conditions, such electrolytes and processes have not received widespread commercial acceptance as a direct result of one or more problems associated with their practice. One problem relates to the sensitivity of such electrolyte systems to the presence of contaminants such as cleaners, salts of nickel plating solutions, chromium plating solutions and zinc metal ions, all of which are frequently introduced into the electrolyte during conventional commercial practice. Such contaminants are introduced into the solution as iron from racks used to hold the parts during plating; as contaminants from inadequate rinsing of the parts prior to entry into the baths; or from etching of the parts themselves during immersion in such baths.
With the advent of new techniques such as shown in commonly assigned U.S. Pat. Nos. 4,462,874; 4,469,569; and 4,933,051, these cyanide-free plating processes have achieved a significant advance in commercial acceptance. However, even with these improvements in the cyanide-free plating art the copper baths eventually are subject to contaminants which will cause the baths to be unusable even though significant levels of copper (II) ions remain in solution along with significant quantities of rather expensive complexing agents. Thus, contaminants such as iron, tin, lead and others detrimentally affect the solutions and require premature disposal of useful portions of the solution unless the solution can be somehow purified or replenished.
Strong oxidizing agents such as U.S. Pat. No. 3,833,486 seem to extend the life of such contaminated solutions however, these also have practical limits. Thus, in order to increase the efficiency and commercial acceptance of these processes, it has been an unrealized goal in the art to remove the undesirable contaminants from such plating baths to extend the useful life of these baths.
Thus, U.S. Pat. Nos. 4,600,493; and 4,762,601 disclose use of a dialysis cell to separate contaminants from useful metal ions or cations. However, such a dialysis cell is extremely expensive to operate. Additionally, while the metal ion or cation component is a desirable component of the bath, it is also desirable to save the expensive complexing agents such as HEDP in such solutions. Use of dialysis cells does not result in the ability to separate these complexing agents. Other processes which attempt to separate useful components from copper plating baths include U.S. Pat. No. 5,006,262 which discloses a process for recovering copper from copper iron baths by reducing cupric (Cu.sup.+2) ions to metallic copper (Cu.sup.0) for recovery of copper from such baths prior to disposal of the baths. While copper is removed from the baths it is not readily useable in other plating baths in the reduced form and thus is not a commercially practical process for replenishing a plating bath. Additionally, there is no means for removal of the complexing agent which is an extremely expensive component of such plating baths. Other processes which relate to methods of purification of copper baths include U.S. Pat. Nos. 4,352,786; 4,162,217; 4,144,149; 4,107,011; 4,072,605; 3,666,447; 3,658,470; and 3,360,447. While many of these prior art patents disclose useful processes most are not readily reusable in electroplating baths and require further steps to utilize the copper or the like recovered from such baths. Additionally, these lack the advantage of removing complexing agents from such solutions in a readily reusable form.
Therefore, it has been a goal in the art to provide a suitable method for addressing the problem of unusable cyanide-free copper baths to allow the separation and use of useful components remaining in the bath from the unusable and detrimental contaminants in such a bath.